I am configuring the Sam3x8e to enter wait mode and wake from wait mode on an RTT interrupt (every 8 Hz, this is working fine).
However, I also want the micro to wake when UART activity occurs on wake up pin 4. The micro correctly wakes when activity occurs on WKUP4 however I need a way to determine that the wake up occurred due to activity on the WKUP4 pin and not because the RTT interrupt fired.

Based on the data sheet, I thought that polling the SUPC_SR register would tell me if the wake up pin was active. However, the value of this register never changes.

Is there a different status register which states which wake up pin triggered the wake up, or better is there an interrupt I can configure to catch the wake up event?

The reason I wish to know where the wake up event came from (wake up pin or RTT interrupt) is because if it came from the wake up pin I want to keep the micro from entering wait mode for a given period of time to ensure I successfully receive the UART packet.